1. Field of the Invention
The invention relates to a method and apparatus for obtaining optical heterodyne reception for DPSK or ASK-modulated transmission signals.
2. Description of the Prior Art
An optical heterodyne reception wherein a supplied optical transmission signal is superimposed with an optical local oscillator signal and the output signal of the receiver is obtained from the superimposed optical signals by way of an intermediate frequency filter. The receiver sensitivity is generally dependent on the difference between polarization conditions and the phases of the transmission signal and of the local oscillator signal. When the polarization conditions of the transmission signal and local oscillator signal do not coincide losses of sensitivity during reception occur. When using a local intermediate frequency that is one or more orders of magnitude lower as compared to the data rate transmitted by the transmission signal, the optical signal superimposed on each other also change periodically between co-phase and quadrature components.
Prior art polarization diversity receivers are known wherein the transmission signal is divided into two polarization conditions orthogonal relative to each other and these are separately received and are electrically added. Such receivers can be employed for polarization-insensitive reception.
Phase diversity receivers wherein the co-phasal and quadrature components are separately received and electrically added can be utilized for phase-insensitive receivers.
These known receivers require a 90.degree. hybrid which is difficult to realize in optical systems and also require two or four optical heterodyne receivers.
Phase-insensitive and/or polarization-insensitive receivers could also be achieved with the phase-controlled and/or polarization control. However, such controls are extremely complicated.